A desirable type of photodetector is a two-color infrared radiation (IR) detector having simultaneous sensitivity in two spectral bands. The spectral bands may include short wavelength IR (SWIR), medium wavelength IR (MWIR), long wavelength IR (LWIR), and very long wavelength IR (VLWIR). An array of two-color IR detectors may be employed in a number of imaging applications wherein it is required to simultaneously detect radiation within two spectral bands from a scene within a field of view of the array. By example, the array may simultaneously detect LWIR and MWIR, or LWIR and SWIR.
FIG. 1 illustrates a two-color detector of type similar to that disclosed in commonly assigned U.S. Pat. No. 5,113,076, issued May 12, 1992, entitled "Two Terminal Multiband Infrared Radiation Detector" to E. F. Schulte. This commonly assigned patent discloses a radiation detector having two heterojunctions that function in a manner analogous to two back-to-back photodiodes. Each of the photodiodes is responsive to radiation within a different IR spectral band, such as LWIR and MWIR. Detection of a particular wavelength band is achieved by switching a bias supply. Disclosed configurations include an n-p-n configuration, a p-n-p configuration, and a p-n-p-n configuration.
Reference is also made to commonly assigned U.S. Pat. No. 5,149,956, issued Sep. 22, 1992, entitled "Two-Color Radiation Detector Array and Methods of Fabricating Same", by P. R. Norton. This patent describes the formation of a substantially continuous common layer between semiconductor regions responsive to different wavelength bands (e.g., MWIR and LWIR). A contact 28 is made to the common layer for coupling same to readout electronics.
Reference is also made to commonly assigned U.S. Pat. No. 5,380,669, issued Jan. 10, 1995, entitled "Method of Fabricating a Two-Color Radiation Detector Using LPE Crystal Growth", by P. R. Norton. This patent describes the use of Liquid Phase Epitaxy (LPE) to grow an n-type LWIR layer, a p-type MWIR layer, and an n-type MWIR layer on a sacrificial substrate. A passivation layer is then formed over the n-type MWIR layer, an IR transparent substrate is bonded to the passivation layer, and the sacrificial substrate is then removed. The resulting structure is then further processed to form an array of two-color detectors.
Also of interest is commonly assigned U.S. Pat. No. 5,457,331, issued Oct. 10, 1995, entitled "Dual-Band Infrared Radiation Detector Optimized for Fabrication in Compositionally Graded HgCdTe", by K. Kosai and G. Chapman (the inventors of the subject matter of this patent application).
Reference can also be made to the n-p+-n dual-band detector described by J. M. Arias et al. in the Journal of Applied Physics, 70(8), Oct. 15, 1991, pps. 4820-4822. In this triple-layer n-p.sup.+ -n structure MWIR absorption occurs in the bottom n-type layer, and LWIR absorption occurs in the top n-type layer.
In U.S. Pat. No. 4,847,489, Jul. 11, 1989, entitled "Light Sensitive Superlattice Detector Arrangement with Spectral Sensitivity" Dietrich discloses a detector arrangement comprising a plurality of photosensitive detector elements. Each of the detector elements has a multilayer structure of alternating positively and negatively doped photosensitive semiconductor material having a superlattice structure. A control voltage is said to control spectral light sensitivity, and an optical filter arrangement is provided for dividing the photodetectors into an upper and lower effective spectral range group.
In U.S. Pat. No. 4,753,684, Jun. 28, 1988, "Photovoltaic Heterojunction Structures" Ondris et al. describe a three layer, double heterojunction Group II-VI photovoltaic structure.
In Japanese Pat. No. 55-101832, Aug. 4, 1980, Makoto Itou discloses, in the Abstract, an infrared detector comprised of n-type HgCdTe having electrodes 2 and 3 arranged on opposite surfaces. A polarity of a bias voltage is switchably coupled to the electrodes 2 and 3. This device is said to enable rays of wide wavelength ranges to be detected by only one semiconductor detector.
General information regarding IR-responsive materials may be found in an article entitled "HgCdTe and Related Alloys", D. Long and J. L. Schmit, Semiconductors and Semimetals, Vol. 5, IR Detectors, Academic Press 1970.
An article entitled "Some Properties of Photovoltaic Cd.sub.x Hg.sub.1-x Te Detectors for Infrared Radiation", by J. M. Pawlikowski and P. Becla, Infrared Physics, Vol. 15 (1975) pp. 331-337 describes photovoltaic p-n junction detectors constructed of HgCdTe crystals and epitaxial films. The authors report that the position of a photosensitivity maximum is shifted within a spectral region of 1-9 microns by changing a molar fraction of cadmium.